Memory medium

ABSTRACT

Disclosed is a method of recording/reproducing information signals at an access speed in the order of μs, a recording density of 1 to 10 GBs/cm 2 , and a data transfer rate in the order of Gbit/sec without breakage of the data. The method includes the steps of: making a head device face to a memory medium having a flat recording surface, the head device including a plurality of head elements two-dimensionally arranged each of which has at its leading end a flat portion having an area of 0.1 μm 2  or less; moving the head device relative to the memory medium a distance more than a gap between two adjacent ones of the head elements; and recording an information signal at a specific position of the recording surface at a recording density of 1 Gbit/cm 2  or more, or reproducing an information signal previously recorded on the recording surface at a specific position by the head device.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method of recordinginformation signals in digital form on a memory medium at a relativelyhigh density or reproducing (or detecting) information signalspreviously recorded in digital form on a memory medium at a relativelyhigh density, and a head device and a memory medium used for therecording/reproducing method.

[0002] Various kinds of large-capacity memories are presently known, forexample, a semiconductor memory represented by a DRAM or Flash memory, amagnetic tape represented by a video recorder, and a disk memoryrepresented by a compact disk or hard disk. These memories havingproblems, for example, in terms of high bit cost and low access speedmay be not suitably used for the future information inputting/outputtingapparatuses such as a microprocessor or network requiring the moreincreased data transfer rate and data capacity. A related art hard disk,optical disk or magnetic tape is lower two digits or more in cost perunit data (bit) than a semiconductor memory; however, it issignificantly inferior in access time, data transfer time and volume ofthe disk to the semiconductor memory.

[0003] At present, with the enhanced performances of computers and theincreased communication speed of information networks, the quantity ofdata to be processed has become larger and the processing rate of datato be processed has become high. To meet such technical development, itis desired to realize a read only memory and a writable memory with thecost per bit kept substantially comparable to that of a magnetic disk oroptical disk and with the access time, data transfer time and volume ofthe memory increased to the levels comparable to those of asemiconductor memory.

[0004] The size of a semiconductor chip, for example, used in a DRAM hasbecome larger with the progressing technical generation, and it isexpected that the size of a semiconductor chip be more than about 3×3 cmat the 4 Gbit-generation. In this case, the area including a packagewill be about 12 cm². To be used like such a DRAM, a memory to bedeveloped is desired to have a size smaller than the above value and alow bit cost.

[0005] The memory capacity stored in the above-described area (about 12cm²) is preferably equivalent to a capacity, for example, which iscapable of storing dynamic images for about one hour, and morespecifically, the memory capacity is required to have about 12 Gbits,that is, a memory density of at least 1 Gbit/cm² in consideration ofdigital image signals with the compressed frequency band.

[0006] As the memory capable of meeting the above-described requirement,there have been extensively studied memories of a type using a so-calledSPM (Scanning Probe Microscope) such as a STM (Scanning TunnelingMicroscope) or AFM (Atomic Force Microscope).

[0007] Such a memory has been described in detail, for example, in “H.J. Mamin et al.: IBM J. Res. Develop. Vol. 39, 681 (1995)”. This memorydetects an information signal using a head device 100 shown in FIG. 10A.The head device 100 has a beam cantilevered with its one end fixed on ahead substrate 102, which is generally called a cantilever 103, and ahead element 101 as a signal detecting portion (hereinafter, referred tosimply as “head element”) formed at the leading end of the cantilever103. The head element 101 is sharply pointed into the shape of atriangular or quadrangular prism by a semiconductor process. The leadingend of the head element 101 sharply pointed up to the level of atomicsize is moved close to the surface of a substance to be measured (datasurface in the case of the memory), and an interatomic force actingbetween the head element 101 and the surface of the substance or atunneling current flowing therebetween is directly or indirectlymeasured, to thus obtain information therefrom.

[0008] B. D. Terris et al. have reported in “Appl. Phys. Lett. 69(27),4262(1996)” a method of preparing a data patten applicable to adisk-like medium by an electron beam plotting apparatus, transferringthe patten on an ultraviolet cured resin layer formed on a glass disk bya so-called glass 2P process to prepare a data disk, and reproducingdata signals stored in the disk by an AFM.

[0009] H. J. Mamin et al. have reported in “Sensors and Actuators A48,215(1995)” a method of bringing a leading end of an AFM in contact witha high polymer substrate, heating the leading end of the AFM by laser tomelt the surface of the high polymer substrate, thereby recording dataon the surface of the high polymer substrate, and reproducing the dataat a reproducing rate as high as 1 Mbit/sec by the AFM.

[0010] The apparatus for recording/reproducing information signals ineach of these documents carries out recording/reproducing with the diskmedium rotated using one AFM head.

[0011] H. Kado et al. have reported in “Appl. Phys. Lett. 66(22),2961(1995)” a method of forming a platinum thin film on a siliconsubstrate and also forming an amorphous GeSb₂Te₄ film thereon, carryingout recording by applying a pulsive electric field between a sharpenedconductive head element and the platinum thin film, and carrying outreproducing by detecting a difference in electric conductivity as achange in current.

[0012] The above detection of data using the AFM, however, is notsuitable for reproducing information signals at a high rate because theinteratomic force is converted into a mechanical displacement of thecantilever 103 and the displacement is detected by a displacement meterusing a piezoelectric effect or laser. Also in the case whereinformation signals are recorded or reproduced on or from a rotatingdisk-like recording medium, the above detection of data using the AFM isdisadvantageous in that it takes a time to wait rotation and the accessspeed becomes low.

[0013] To improve an effective data transfer rate, there has been knowna method of carrying out parallel processing using a plurality of headdevices. For example, S. C. Minne et al. have reported in “Appl. Phys.Lett. 67(26), 3918(1995)” an apparatus in which two AFM head deviceswith leading ends of head elements separated 100 μm from each other arearranged in parallel whereby images of a grating with a cycle of 5 μmare reproduced. In this parallel processing apparatus, ZnO having apiezoelectric effect is used for part of each cantilever to individuallydisplace the two cantilevers in the depth direction, and the size of thecantilever becomes larger (length: 420 μm, width: 85 μm) forsufficiently ensuring the displacement, with a result that themechanical resonance frequency becomes low to reduce the data transferrate. Accordingly, even in the case of using a plurality of the AFM headdevices, the data transfer rate is not improved so much.

[0014] The data reproducing apparatus (microscope) using a plurality ofthe head elements described in the above document does not report amethod or mechanism of detecting or correcting a positional relationshipbetween each head element and desired data in the direction parallel tothe surface to be measured, causing a problem that the addressmanagement for data which is important for the memory apparatus cannotbe performed. Even if each distance and positional relationship betweentwo pieces of respective head elements has been clearly measured andalso the positional relationship between either one of the head elementsand the address of the data surface has been measured, the relationshipbetween each head element and the data position cannot be kept resultingfrom a difference in thermal expansion coefficient between the headarray and the substrate of the recording medium, for example, caused bytemperature change.

[0015] In the memory apparatus using the SPM, since the leading end ofthe head element 101 of the head device 100 is very sharpened as shownin FIG. 10B and only the leading end of the head element 101 is broughtin contact with the data surface, if an impact force is applied to thememory apparatus during reproducing of data, the data surface in contactwith the leading end of the head element 101 may be applied with a veryhigh local pressure, which causes a fear of breakage of data stored inthe data surface. For example, S. C. Minne et al. have reported in“Sensors and Actuators A48, 215 (1955)” that a leading end of a headelement is formed into a spherical shape having a curvature of 100 nm orless. A spring constant of the cantilever is in the order of 1 N/m. Now,it is assumed that a leading end of a head element applied with animpact force is displaced 10 nm on the data surface side and is broughtin contact with the data surface; and the leading end of the headelement is formed into a flat circular shape having a radius of 10 nm.In this case, a pressure applied to the flat circle portion is as veryhigh as 3×10⁷ N/m², which will cause breakage data stored on the datasurface of a medium, insofar as the medium is made from a usualmaterial. Even if there does not occur breakage of data, since theleading end of the head element is worn, there arises another problemthat the shape of the leading end is changed and thereby the resolutionin recording or reproducing is reduced.

[0016] In the above-described recording/reproducing method proposed byH. Kado, since a current value upon reproducing is 10 pA at a locationwhere information signals have been not recorded and is 1 nA at alocation where information signals have been recorded, a reproducingsignal with a sufficient S/N can be obtained when the data reproducingrate is low because the frequency band of the reproducing signal isnarrow; however, a reproducing signal with a sufficient S/N cannot beensured at the above small signal current of about 1 nA when the datareproducing rate is high because the frequency band of the reproducingsignal becomes wide.

SUMMARY OF THE INVENTION

[0017] An object of the present invention is to provide a method ofrecording/reproducing information signals on/from a ROM and a writablememory medium at an access speed in the order of μs, a recording densityof 1 to 10 GBs/cm², and a data transfer rate in the order of Gbit/secwithout breakage of the data.

[0018] To achieve the above object, according to the present invention,there is provided a method of recording/reproducing an informationsignal, including the steps of: making a head device face to a memorymedium having a flat recording surface, the head device including aplurality of head elements two-dimensionally arranged each of which hasat its leading end a flat portion having an area of 0.1 μm² or less;moving the head device relative to the memory medium a distance morethan a gap between two adjacent ones of the head elements; and recordingan information signal at a specific position of the recording surface ata recording density of 1 Gbit/cm² or more, or reproducing an informationsignal previously recorded on the recording surface at a specificposition by the head device.

[0019] As described above, according to the method of and apparatus forrecording/reproducing information signals in accordance with the presentinvention, there can be obtained a ROM system and a writable memorysystem having an access speed in the order of μs, a recording density of1 to 10 GBs/cm², and a data transfer rate in the order of Gbit/s.

[0020] The information signal recording/reproducing apparatus of thepresent invention is inferior in processing rate and reliability to asemiconductor memory but is lower two digits or more in bit cost thanthe semiconductor memory, and therefore, it can be applied to a varietyof technical fields. That is to say, the recording/reproducing apparatusof the present invention may be used in applications including a CD, MD,electronic game machine, DVD and video camera, in place of an opticaldisk and a magnetic disk, particularly, it may be effectively used for aportable CD player called a walkman, digital camera or may be used inplace of a hard disk for a lap top type computer or the terminal of aportable computer.

[0021] The method of and apparatus for recording/reproducing informationsignals in accordance with the present invention are also veryeffectively used for applications which require processing of a largeamount of data, high speed retrieval, and high speed access, forexample, for a recognition data base for video recognition and imagerecognition and a data bank for video demand distribution.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a side view, with an essential portion enlarged, showingthe shape of a leading end of a head element as a first embodiment ofthe present invention;

[0023]FIG. 2 is a side view, with an essential portion enlarged, showingthe shape of a leading end of a head element as a second embodiment ofthe present invention;

[0024]FIG. 3 is a side view, with an essential portion enlarged, showingthe shape of a leading end of a head element as a third embodiment ofthe present invention;

[0025]FIG. 4 is a side view, with an essential portion enlarged, showingthe shape of a leading end of a head element as a fourth embodiment ofthe present invention;

[0026]FIG. 5 is a conceptional view illustrating a method ofrecording/reproducing information signals according to the presentinvention;

[0027]FIG. 6 is a conceptional plan view of a memory medium suitablyused for the method of recording/reproducing information signalsaccording to the present invention;

[0028]FIG. 7 is a conceptional plan view of a head device suitably usedfor the method of recording/reproducing information signals according tothe present invention;

[0029]FIGS. 8A to 8H are process diagrams illustrating a method ofmanufacturing the head device as the first embodiment of the presentinvention;

[0030]FIGS. 9A to 9D are process diagrams illustrating a method ofmanufacturing the head device as the second embodiment of the presentinvention; and

[0031]FIGS. 10A and 10B are conceptional views showing a related arthead device, wherein FIG. 10A is a side view of the head device and FIG.10B is a side view, with an essential portion enlarged, showing a headelement of the head device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] Hereinafter, embodiments of the present invention will bedescribed with reference to the accompanying drawings.

[0033] Referring first to FIGS. 1 to 4, there will be described varioushead elements of the present invention.

[0034]FIG. 1 is a side view, with an essential portion enlarged, showingthe shape of a leading end of a head element as a first embodiment ofthe present invention; FIG. 2 is a side view, with an essential portionenlarged, showing the shape of a leading end of a head element as asecond embodiment of the present invention; FIG. 3 is a side view, withan essential portion enlarged, showing the shape of a head element as athird embodiment of the present invention; and FIG. 4 is a side view,with an essential portion enlarged, showing the shape of a leading endof a head element as a fourth embodiment of the present invention.

[0035] First, the head element as the first embodiment of the presentinvention will be described with reference to FIG. 1.

[0036] As described above, the leading end of the related art headelement, shown in FIG. 10B, is pointed sharply enough to allow detectionof a change at the atomic size level, and is regarded as asemi-spherical shape having a very small curvature in an enlarged view,so that when the leading end of the related art head element is broughtin contact with a memory medium, the contact point of the memory mediumis locally applied with a very high pressure, causing a fear of breakageof data stored on the memory medium. The head element shown in FIG. 1 isintended to solve such a disadvantage.

[0037] In FIG. 1, reference numeral 1A indicate the head element of thepresent invention. To reduce a pressure applied to a memory medium atthe time of the above collision, a leading end 2 of the head element 1Aas an information signal detecting portion or an information signaldetecting electrode is flattened as indicated by reference numeral 3.Taking it into account to detect an information signal recorded at adensity of 1 Gbit/cm² or more, the size of the leading end of the headelement 1A is required to be in a range of 0.1 μm² or less.

[0038] The larger the area of the flat portion 3 becomes, the more theforce generated at the collision with a memory medium is dispersed. As aresult, the probability of breakage of data is reduced. For example, inthe case where the cell size of data to be reproduced (detected) is0.1×0.1 μm and it is judged whether the data is “1” or “0” on the basisof the presence or absence of a recess of the cell size, the spacialresolution upon reproducing may be set at a value in the order of thecell size. Assuming that the resolution is set at a value being a halfof the cell size, that is, 0.05×0.05 μm, the size of the flat portion 3of the leading end 2 of the head element 1A may be set at a value in thesame order, that is, 0.05×0.05 μm. The contact area of the flat portion3 is a square having one side of 0.05 μm, although the contact area ofthe leading end of the related art head element is a circle having aradius of 10 nm. That is, the area ratio therebetween becomes 8, andthereby the impact pressure applied to the flat portion 3 upon collisionis reduced to one-eighth of the impact pressure applied to the leadingend of the related art head element upon collision. It is to be notedthat the wording “flat” means not only a perfect flat state but also asurface somewhat coarsened or a shape having a curvature similar to orsmoother than that of the data cell size.

[0039] Although only the flattening of the leading end of the headelement exhibits an effect of reducing the impact upon collision, thestructure of a head element 1B as the second embodiment shown in FIG. 2is more effective, in which a peripheral portion 4 of a conductiveleading end 2 is made from a non-conductive material, whereby thecontact area with a memory medium upon collision is extended withoutsubstantially extending the area of a flat portion 3 of the leading end2 as the information signal recording portion or detecting portion.

[0040] The leading end of the head element may be formed into a shapedifferent from a prism or needle as in the related art AFM. Concretely,as shown in FIG. 3, a head element 1C as the third embodiment of thepresent invention is configured such that a thin film as a leading end 2is formed on the surface of a substrate 5 made from a non-conductivematerial, whereby the flatness of the leading end 2 can be more easilyensured.

[0041] The shape of the leading end 2 of each of the head element 1Cshown in FIG. 3 and the head element 1B shown in FIG. 2 is advantageousas follows: namely, in the case of reproducing a memory medium on whichinformation signals are recorded (stored) in the form of irregularities,the leading end 2 is prevented from being erroneously bitten in eachrecess of the irregularities of the memory medium because the flatportion 3 of the leading end 2 can be wider than the area of the recess.This is effective to essentially prevent an error of a reproducingsignal or mechanical breakage of the data surface caused by erroneousbiting of the leading end 2 in the recess.

[0042] A head element 1D as the fourth embodiment of the presentinvention shown in FIG. 4 is configured such that a leading end 2 havinga columnar structure is formed on the surface of a substrate 5. Withthis structure, even if the edge of the leading end 2 is worn, the sizeof the leading end 2 is not changed, whereby the spacial resolution inrecording or reproducing is not reduced.

[0043] In the case where the area of the flat portion of the leading endof each head element is enlarged to make wider the contact area with amemory medium as described above, there is a possibility that the datasurface of the memory medium is degraded by friction and wear. Toprotect of the data surface of the memory medium or the leading end 2 ofthe head element, either or both of them are preferably formed with aconductive material having a small friction coefficient and a largehardness. For example, either or both of them may be covered with a filmmade from carbon or carbon hydride (diamond like carbon) by a sputteringor CVD process. It is effective that such a film is further thinlycoated with a lubricant made from a high polymer material. The highpolymer lubricant preferably has a conductivity; however, even if thehigh polymer lubricant is insulating, it allows reproducing using accurrent insofar as it has a thin film thickness.

[0044] A method of increasing a data transfer rate upon reproducing datawill be described below.

[0045] For reproducing a large-capacity of data instantly, it isrequired to increase a data transfer rate as well as an access speed.

[0046] To increase a data transfer rate, it is important to ensure asufficient S/N even in a wide frequency band, and to adopt a reproducingmethod which is essentially not limited to the data transfer rate, forexample, a reproducing method limited to the resonance frequency of theapparatus. In the case of the AFM or STM, the spacial resolution is veryhigh; however, the output is dependent on a very weak force or currentsuch as an interatomic force or tunneling current, so that it isdifficult to obtain a large reproducing signal and hence to obtain asufficient S/N in a wide frequency band.

[0047] In view of the foregoing, according to the present invention,there is adopted a method of reproducing information signals in whichthe data “1” or “2” is made to correspond to switching of a current andreproducing is performed by turn-on/turn-off of the current.

[0048]FIG. 5 is a conceptional view of an information signalrecording/reproducing apparatus as an embodiment for illustrating themethod of recording/reproducing information signals according to thepresent invention; FIG. 6 is a conceptional plan view of a memory mediumsuitably used for the method of recording/reproducing informationsignals according to the present invention; and FIG. 7 is a conceptionalplan view of a head device suitably used for the method ofrecording/reproducing information signals according to the presentinvention.

[0049] In FIG. 5, reference numeral 10 indicates an information signalrecording/reproducing apparatus as the embodiment of the presentinvention. The recording/reproducing apparatus 10 includes a memorymedium 20 shown in FIG. 6, a head device 30 shown in FIG. 7, a powersupply 11, and an amplifier 12.

[0050] It is to be noted that in the specification the wording“recording/reproducing apparatus” means not only an apparatus havingboth the “recording and reproducing” functions but also an apparatushaving only the “recording” function or the “reproducing” function.

[0051] The memory medium 20 of the present invention has a structureshown in FIGS. 5 and 7 in which a conductive layer 22 is formed on aquadrilateral shaped flat conductive substrate 21. The recording surfaceof the conductive layer 22 is, as shown in FIG. 6, sectioned into aplurality of sectors 23 arranged, for example, in a cross-cut pattern.One sector 23 is a unit data area which allows reproducing by a specificone head element 1. On each sector 23 are recorded information signalsin the form of irregularities.

[0052] The head device 30 is so configured as shown in FIGS. 5 and 7 inwhich a number of head elements 1 (of a type selected from the headelements 1A, 1B and 1C) are arranged, on a substrate 5, in a specifictwo-dimensional pattern, for example, in such a matrix pattern that thehead elements 1 correspond to the sectors 23 of the memory medium 20 ona one-for-one basis in the X-direction and the Y-direction with thedistance (pitch) between at least the adjacent heads on the X-directionset at a value of Pa and with the head pitch in the Y-direction set at avalue of Pb. That is to say, one head element 1 operates in one sector23 as a recording region or reproducing region.

[0053] The method of reproducing information signals according to thepresent invention using the recording/reproducing apparatus 10 of thepresent invention will be described below.

[0054]FIG. 5 shows a principle of reproducing information signalspreviously recorded in the form of irregularities on each sector 23 ofthe memory medium 20 in a state in which the head device 30 is broughtin contact with the memory medium 20. Referring to FIG. 5, an electricfield is applied between the conductive substrate 21 of the memorymedium 20 and the head element 1 by the power supply 11. While notshown, a mechanism is provided for suitably bringing the head element 1in contact with the memory medium 20, and only when the head element 1is brought in contact with the projection of the irregularities of thememory medium 20, a current is allowed to flow from the head element 1to the memory medium 20. Such a current is current-voltage converted andamplified by the amplifier 12 into a large signal voltage.

[0055] In the case where the contact area of the head element 1 incontact with the memory medium 20 is 0.05×0.05 μm, assuming that thecurrent density is 5×10⁸ μ/m², a current of 1.4 μA flows only when thehead element 1 is brought in contact with the projection of the memorymedium 20, and assuming that the resistance upon current-voltageconversion is 500 hm, the current of 1.4 μA is current-voltage convertedinto a voltage of 70 μV. Accordingly, a sufficient S/N can be obtainedeven for a reproducing signal frequency band of 10 MHz.

[0056] In the case of using the resistance of 500 hm uponcurrent-voltage conversion, a thermal noise of the resistance becomespredominant as the noise in the ideal case. Assuming that the signalfrequency band is 10 MHz and the temperature is 300 K, the thermal noisecurrent is calculated into a value of 57 nA. The signal current of atleast 200 nA, preferably, 570 nA is required for reproducing a digitalsignal in a state not being obstructed by the noise current. In the caseof reproducing information signals at the above current density of 5×10⁸A/m², the contact area of the head element 1 in contact with the memorymedium 20 becomes at least 0.0004 μm², preferably, 0.0011 μm², andaccordingly, if the contact portion of the head element 1 has a squareshape, the size of the contact portion becomes at least 0.02×0.02 μm,preferably, 0.033×0.033 μm.

[0057] In the embodiment shown in FIG. 5, a dc electric field is appliedto the head element 1; however, an ac electric field may be appliedthereto. In the case where an extremely thin insulating layer is formedon the head element 1 or on the reproduced surface of the memory medium20 or in the case where a space such as an extremely thin air layer isformed between the head element 1 and the reproduced surface of thememory medium 20, it is preferred to apply an ac electric field having ahigh frequency.

[0058] In the case of using a focus mechanism for bringing the headdevice 30 in contact with the memory medium 20 using an electrostaticforce, there is a fear of interference between the electric fieldrequired for signal detection and the electric field required for thefocus mechanism. To avoid such an inconvenience, it is preferred to use,for signal detection, an ac electric field having a frequency veryhigher than the resonance frequency of a movable portion of the headdevice 30.

[0059] In the above-described document “H. Kado et al.: Appl. Phys.Lett. 66(22), 2961(1995)”, information signals are reproduced from thewritable memory medium made from amorphous GeSb₂Te₄ by making use of achange in electric conductivity between states before and afterrecording. In this technique, since the reproducing signal current is asweak as 1 nA, it is difficult to carry out reproducing at a high rate.The reason why the reproducing signal current is weak is that if thereproducing current is large, the conductivity of the non-recordedportion of the memory medium 20 will be changed, that is, the recordingwill be performed, and consequently the reproducing current cannot beset at a large value. To make large the reproducing current, it isrequired to make large a current value to start recording on thenon-recorded portion. To meet such a requirement, it is effective tochange the composition of the material, film thickness, or the shape ofthe head element.

[0060] As described in the above embodiment, for the read only memorymedium 20, the possibility of recording is very small and thereby theupper limit of the reproducing current can be increased. For a writabletype memory medium, control of the current to start recording differsdepending on the recording mechanism. In general, the recordingmechanism is classified into a type in which recording is performed byheat generated due to flow of a current, a type in which recording isperformed by an electric field locally increased, and a type in whichrecording is performed by a pressure generated upon collision between ahead element (probe) and a memory medium. In each type of the recordingmechanism, the sharper the leading end of the head element (probe), thesmaller the current to start recording. Accordingly, it is difficult toobtain a large reproducing current. From this viewpoint, the leading endof the head element (probe) is desired to a flat structure which allowsa reproducing signal current of 10 nA or more.

[0061] For carrying out rapid access, it is desired to make the movableportion as small and light as possible and also to make the movingdistance as short as possible. A disk medium utilizing apparatus such asa hard disk apparatus or optical disk apparatus generally mounts onehead per one recording surface and moves the disk medium in the radialdirection thereof by a swing arm or linear actuator. It may beconsidered that a number of magnetic heads or optical heads are arrangedin the radial direction to reduce the moving distance per one head,thereby improving the access speed to the desired radial position;however, this method has disadvantages that the magnetic head or opticalhead is high in cost per one head and also the volume is large andthereby the size of the memory apparatus is increased. Actually, themethod of improving the access speed by arranging a number of magneticheads or optical heads has been not adopted.

[0062] On the contrary, since the head device 30 of the presentinvention is extremely small and a plurality of the head elements can beeasily manufactured (as will be described later), the head device 30 canbe configured to have a multi-head as shown in FIG. 5. The head device30 of the present invention can be also configured such that a pluralityof heads are arranged in the radial direction of a rotating memorymedium such as a disk, whereby the heads can be accessed to all of theinformation signals recorded on the memory medium by moving each head atleast a distance more than a pitch between the adjacent heads in theradial direction of the memory medium.

[0063] In the above configuration, assuming that the head pitch is 1 mm,the heads can be instantly accessed to desired tracks only by movingthem a distance of 1 mm.

[0064] Even in the case of adopting such a configuration, however, for arotating type disk medium, it takes a time to wait rotation. To copewith such an inconvenience, it may be considered to rotate the diskmedium at a high speed; however, this method has a problem in terms ofstability and reliability of a spindle motor.

[0065] According to the present invention, as shown in FIGS. 5 to 7, theaccess speed is improved by bringing the head device 30 having aplurality of the head elements 1 arranged in a matrix pattern in contactwith the recording surface of the memory medium 20, and moving the headdevice 30 relative to the memory medium 20 a pitch P of the head element1.

[0066] The head device 30 can be moved in the X and Y directions bymoving a movable stage on which the head device 30 is mounted, forexample, using a stepping motor, DC motor, or piezoelectric actuator.

[0067] In the case where the pitch P between the head elements 1 is 2mm, that is, in the case where the arrangement density of the headelements 1 of the head device 30 is 25 head elements/cm², a movementdistance to a data position is about 1 mm. In this case, assuming thatan average driving speed of the actuator is 1 m/s, the access speedbecomes 1 ms which is faster several times than the related art one. Ifthe pitch P between the head elements is made shorter, for example, to0.2 mm, the access speed can be made fast to 100 μs.

[0068] As described above, in the case of reproducing informationsignals recorded on the memory medium 20 using the head device 30including a plurality of the head elements 1, data stored on a pluralityof the sectors 23 of the memory medium 20 can be simultaneouslyreproduced by the plurality of the head elements 1. For a longinformation signal, it is undesirable to continuously reproduce it byone head element 1. Such a long information signal may be first dividedinto parts and recorded on a plurality of different ones of the sectors23 on the memory medium 20. In this case, the long information signalcan be reproduced for a short time by simultaneously scanning aplurality of ones of the head elements 1 correspondingly to theplurality of ones of the sectors 23 on which the parts of the longinformation signal are recorded.

[0069] In addition to the reproducing rate, the head device 30 hasanother advantage. When an information signal is divided into parts andrecorded on a plurality of ones of the sectors 23 as described above,even if a part of the information signal stored on a certain sector 23is broken or a certain head element 1 is broken down, it is possible toeliminate such a fear that all of the parts of the information signalcannot be reproduced, and to carry out reproducing with no error usingsuitable error collection. As a result, it is effective to divide aninformation signal into parts and record the parts on different ones ofthe sectors 23.

[0070] A plurality of recording/reproducing apparatuses 10 may beprepared by changing the combination of the head device 30 and thememory medium 20 for each recording/reproducing apparatus, wherein aninformation signal in the same information signal series is dispersedlyrecorded on or reproduced from different ones of the plurality ofdifferent recording/reproducing apparatuses 10, to thereby improve thereliability or improve the effective data transfer rate.

[0071] The addressing method using the above recording/reproducingapparatus 10 will be described below.

[0072] To reproduce a desired information signal, it is necessary toconfirm at what position the desired information signal is recorded onthe memory medium 20 and access the head device 30 to the position, andmore specific, it is necessary to confirm positional information of thedesired information signal and move the head element 1 to the position(in the in-plane direction and the depth direction). Here, thepositional adjustment in the in-plane direction (X, Y direction) iscalled “positioning” and the positional adjustment in the depthdirection is called “focusing” for the sake of convenience.

[0073] To make each head element 1 of the head device 30 face to thecorresponding sector 23 of the memory medium 20 and carry outpositioning of the head element 1 to the sector 23 upon recording orreproducing, it is desired to move the entire head device 30 or move thememory medium 20 with the head device 30 fixed. For carrying out thepositioning, each head element 1 must be moved in the in-plane directiona distance of about the pitch P between the head elements 1. Forexample, in the case where the pitch P between the head elements 1 is0.1×0.1 mm, each head element 1 can be moved a distance of 0.1 mm ormore. In the case of independently positioning the head elements 1,there must be provided actuators each capable of moving each headelement 1 a distance of 0.1 mm or more. That is to say, each mechanismmovable the pitch P between the two adjacent head elements 1 must beformed within a gap between the two adjacent head elements 1. This isdifficult to be realized. On the contrary, there is no problem in thecase of moving the entire head device 30 a distance of about 0.1 mm.

[0074] The movement of the entire head device 30, however, isdisadvantageous in carrying out fine positioning between each headelement 1 and the position of a desired information signal. This isbecause the movement of the entire head device 30 makes it impossible tocorrect an error of the pitch P between the head elements 1 caused atsteps of manufacturing the head device 30, an error of the intervalbetween information signals caused at steps of manufacturing the memorymedium 20, or an error of positioning caused by a difference in thermalexpansion coefficient between the memory medium 20 and the head device30 depending on a change in service environment, for example,temperature and moisture.

[0075] To carry out the error correction (hereinafter, referred to as“fine positioning”), it is effective to provide an actuator capable ofmoving each head element 1 a micro-distance. In the case of the abovepositioning, the head element 1 is moved a distance of 0.1 mm; however,in the case of fine positioning, the head element 1 is sufficient to bemoved a distance of one-tenth or less of the pitch P between the headelements 1. Accordingly, fine positioning can be sufficiently achievedusing a small-sized actuator using a piezoelectric element.

[0076] In this way, the head device 30 of the present invention iscontrolled in two steps: the positioning (course positioning) step formoving the entire head device 30 and the fine positioning step forfinely moving each head element 1.

[0077] In addition to the above two-step control, there is a moresimplified recording/reproducing method in which only the coursepositioning is performed and the fine positioning is not performed. Inthis method, after each head element 1 is substantially moved by coursepositioning to a position where reproducing or recording is to beperformed, information signals on the periphery of the position arecollectively reproduced or recorded; and in the case of reproducing, theinformation signals thus obtained are stored in a buffer memory and thendata at the desired position is specified by signal processing. Forexample, in the case of reproducing information signals along a linecalled a track on which the information signals are recorded, thereproducing is performed without tracking control. In this case, byreproducing information signals at a cycle being twice or more of atleast a cycle (pitch) of the track, the desired information signal canbe restored by the subsequent signal processing. In the case whereinformation signals are two-dimensionally recorded, information signalsmay be reproduced at a cycle being twice of a cycle in each of theX-direction and the Y-direction. Here, the size of the leading end ofthe head element 1 may be set at a large value to make high the spacialresolution and make large a signal current, and more specifically, thelength of the head element 1 is desired to be in a range of one-tenth toone-half of the pitch of the above track.

[0078] The information signals are preferably stored in the buffermemory as digital signals discontinuous with time for ease of thesubsequent signal processing. The sampling cycle for taking signals inthe buffer memory is desired to be a length equivalent of the cell sizeor one bit of the information signal or a time being a half or less thatrequired for movement of the head device by course positioning.

[0079] If both the memory medium 20 and the head device 30 are veryexcellent in terms of smoothness and flatness, focusing may be omitted;however, actually, it is difficult to keep constant the flatness of eachof the memory medium 20 and the head device 30 by the effect ofdeformation due to temperature and moisture or warping due to the filmstress. Assuming that the size of the memory medium 20 is 2×2 mm² andthe angle of warping is 2°, the heights in the depth direction at boththe ends of the memory medium 20 are different 70 μm from each other.Like the above-described positioning case, it is difficult to provideactuator's functions capable of moving the head elements the abovedistance for each head element. Accordingly, even in this case, atwo-step control is effective like the above positioning case.

[0080] To achieve rough focusing, it is desirably simple to flatten boththe head device 30 and the memory medium 20. For this purpose, asubstrate of each of the head device 30 and the memory medium 20 may beformed of a thick plate made from a material having a large elasticmodulus; or the substrate of the head device 30 may be formed of a thickplate made from a less deformable material having a larger elasticmodulus while the substrate of the memory medium 20 may be formed of athin plate made from a deformable material having a small elasticmodulus. Upon recording/reproducing, the above memory medium 20 isbrought in close-contact with the flat stage or the head device 30 ispushed on the memory medium 20 to thus obtain a desired flatness.

[0081] As the less deformable material having a large elastic modulus,there may be used a ceramic material such as glass, or a metal such assilicon, aluminum, or stainless steel. As the deformable material, theremay be used a high polymer such as acrylic resin, polycarbonate, ornylon.

[0082] It is effective to bring the memory medium 20 in close-contactwith the stage or the head device 30 using an electrostatic force.

[0083] To achieve fine focusing, it is desired to mount actuatormechanisms movable about 10 μm or less in the depth directionindependently on respective head elements 1. The actuator mechanism isrepresented by an electrostatic element or piezoelectric element.Whether or not the head element 1 is in contact with the memory medium20 is judged by monitoring an impedance in a reproducing signal line orrecording circuit. Accordingly, the contact of the head element 1 withthe memory medium 2 can be stably feed-back controlled using the signalthus monitored. The fine focusing mechanism may be provided per one headelement 1 as described above, or may be provided per one of a pluralityof adjacent head elements 1.

[0084] In addition to an information signal, an address signal forgiving a relative position of the memory medium 20 to the head element 1or the head device 30 is recorded on the memory medium 20. To be morespecific, the address signal, which is for giving two-dimensionalpositional information in the sector 23, may be previously recorded onthe memory medium 20 in the form of irregularities, or may be recordedon the memory medium 20 by the head device 30 upon recording.

[0085] A method of manufacturing the memory medium 20 will be describedbelow.

[0086] The read only memory medium 20 is preferably configured such thatfine irregularities formed on the substrate surface are used as signals.Like a compact disk, an original plate on which a pattern of fineirregularities is formed by photolithography or using an electron beamplotting apparatus is prepared, and a substrate with the pattern of fineirregularities is formed by injection-molding or extrusion-molding usingthe original plate as a die, to obtain a memory medium.

[0087] Alternatively, a substrate coated with an ultraviolet cured resinis separately prepared, and a pattern of fine irregularities is formedon the resin layer using a so-called 2P (Photo Polymerization) process.

[0088] The substrate may be made from glass or metal as well as a highpolymer such as acrylic resin or polycarbonate.

[0089] The writable memory medium 20 is prepared by preparing a flatsubstrate with no irregularities or a substrate on part of whichirregularities as an address signal are formed by the above-describedmethod, and forming, on the substrate, a material allowing an impedancebetween the head element 1 and the memory medium 20 to be locallychanged before and after local application of an electric field,current, heat, or pressure.

[0090] As such a material, there may be used amorphous GeSb₂T₄ describedin the above-described document, a high polymer dissolved or deformed byheat or pressure, a capacitor for storing electric charges, or aferroelectric material.

[0091] The head element 1 or the head device 30 can be formed on a flatsubstrate made from glass or the like by a semiconductor process. Themovable portion for fine focusing is formed of a membrane supported onthe substrate by means of a cantilever beam or a double-end fixed beamusing a micromachinning process. In addition, ICs such as a focus-servocontroller, head amplifier, and current driver may be integratedlymounted in the substrate of the head device 30.

[0092] An embodiment of a method of manufacturing the head device 30will be described with reference to FIGS. 8A to 8H and FIGS. 9A to 9D.

[0093]FIGS. 8A to 8H are process diagrams illustrating a method ofmanufacturing the head device as the first embodiment of the presentinvention; and FIGS. 9A to 9D are process diagrams illustrating a methodof manufacturing the head device as the second embodiment of the presentinvention.

[0094] First, at the step shown in FIG. 8A, there is prepared asubstrate 31 (equivalent to the substrate 5 of the head device 30)formed of a silicon wafer containing an impurity at a relatively highconcentration (that is, having a conductivity). The substrate 31 may bealready formed with a signal processing circuit or current driver.

[0095] A so-called sacrifice layer having a trapezoidal cross sectionis, at the step shown in FIG. 8B, patterned on-the surface of thesubstrate 31 by a micromachinning process. The sacrifice layer will beremoved later by etching. The material for forming the sacrifice layeris suitably selected from a photoresist, aluminum and SiO₂ inconsideration of the combination with other materials which will beformed in the subsequent steps. Here, a photoresist is patterned byexposure and development. Reference numeral 32 indicates the photoresistpattern.

[0096] Then, at the step shown in FIG. 8C, a film 33 made from analuminum alloy is formed to a thickness of one to several μm. The film33 will be formed into a vertically movable spring member functioning asa fine focusing mechanism.

[0097] The aluminum alloy film 33 is patterned into a pattern 34 at thestep shown in FIG. 8D. The pattern 34 has a plurality of centralportions functioning as a plurality of springs, positioned over anapproximately central portion of the trapezoidal photoresist pattern 32,and a peripheral portion fixed on the periphery of the substrate 31. Inaddition, FIG. 8D-(B) is a sectional view taken on line A-A of FIG.8D-(A).

[0098] The process goes on to the step shown in FIG. 8E at which a finesquare patten of a photoresist 35, having one side less than thethickness of the aluminum alloy film 33, is formed at a central portionof the aluminum alloy film 33 positioned over an approximately center ofthe trapezoidal photoresist pattern 32.

[0099] The aluminum alloy film 33 is isotropically etched usingphosphoric acid at the step shown in FIG. 8F. At this time, the etchingis completed before the square photoresist 35 is perfectly separatedfrom the aluminum alloy film 33 so that the leading end of the aluminumalloy film 33 remains as a flat portion.

[0100] The substrate 31 is annealed in a gas atmosphere containingoxygen at the step shown in FIG. 8G. With this annealing, the stresscaused in the aluminum alloy film 33 can be relieved, and the surface ofthe aluminum alloy film 33 except for the leading end (which is incontact with the micro-photoresist 35) for forming a head element isoxidized to be converted into the insulating surface.

[0101] After annealing, the trapezoidal photoresist 32 and themicro-photoresist 35 are removed by a resist releasing agent at the stepshown in FIG. 8H. As a result, a cavity 6 is formed between the aluminumalloy film 33 and the substrate 31 (equivalent to the substrate 5 shownin FIGS. 3 and 4); a portion of the trapezoidal aluminum alloy film 33over the cavity 6 forms a spring 7; and a head element 1 including aleading end 2 having a flat shape 3 is formed at a central portion ofthe spring 7. It is to be noted that the head element thus formed isindicated by the same reference numeral 1 as that of the head elementshown in FIGS. 3 and 4.

[0102] Finally, the silicon substrate 31 is provided with an electrode,followed by application of a DC bias voltage, and amorphous carbonhydride is selectively formed on the flat portion made from the aluminumalloy having a conductive surface by CVD (Chemical Vapor Deposition)process, to obtain the head element 1.

[0103] Although only one head element is shown in FIGS. 8A to 8H usedfor description of this embodiment, a number of head elements can besimultaneously formed on the substrate 31 and also a number of headdevices 30 can be simultaneously manufactured in accordance with themanufacturing process of this embodiment.

[0104] The method of manufacturing the head device as the secondembodiment of the present invention will be described with reference toFIGS. 9A to 9D. In this manufacturing method, the columnar head element1D shown in FIG. 4 is manufactured.

[0105] First, at the step shown in FIG. 9A, a thick insulating film 42having a thickness of about 1 μm is formed on the surface of a substrate41 which has a conductivity at least on its surface. The insulating film42 may be made from silicon oxide, silicon nitride, photoresist, orpolyimide.

[0106] At the step shown in FIG. 9B, a mask 43 made from a materialwithstanding etching for the insulating film 42 is formed on theinsulating film 42 except for a portion equivalent to the columnarleading end of the head element 1D. In addition, the mask 43 ispreferably formed by X-ray lithography using synchrotron radiation orphotolithography using ultraviolet rays having a short wavelength suchas a ArF excimer laser.

[0107] Then, at the step shown in FIG. 9C, a portion of the insulatingfilm 42 equivalent to the leading end of the head element 1D isanisotropically removed by RIE (Reactive Ion Etching), to form a leadingend forming portion 44.

[0108] After removal of the mask 43, at the step shown in FIG. 9D, ametal such as nickel is formed in the leading end forming portion 44 byplating using the conductive substrate 41 as an electrode, to form aleading end 2. The surface of the leading end 2 thus formed is finishedby polishing to improve the flatness thereof.

[0109] In this way, the head element 1D including the columnar leadingend 2 shown in FIG. 4 is obtained.

[0110] Although only one head element is shown in FIGS. 9A to 9D usedfor description of this embodiment, like the previous embodiment, anumber of head elements can be simultaneously formed on the substrate 41and also a number of head devices 30 can be simultaneously manufacturedin accordance with the manufacturing process of this embodiment.

[0111] While the preferred embodiments of the present invention havebeen described using specific terms, such description is forillustrative purposes only, and it is to be understood that many changesand variations may be made without departing from the scope or spirit ofthe following claims.

What is claimed is:
 1. A head element for recording or reproducing aninformation signal, comprising: a flat portion formed at a leading endof said head element, wherein the area of said flat portion is 0.1 μm²or less and is one-tenth or more of a recording cell size of a memorymedium or the area of one bit.
 2. A head element for recording orreproducing an information signal, comprising: a leading end portionformed into a columnar shape extending longer in the directionperpendicular to the surface of a substrate of said head element.
 3. Ahead element according to claim 1 or 2 , wherein said leading end ofsaid head element comprises a conductive member for recording ordetecting an information signal and a non-conductive member adjacent tosaid conductive member, said non-conductive member being not allowed torecord or detect an information signal.
 4. A head device comprising: aplurality of head elements described in any one of claims 1 to 3 , whichare two-dimensionally arranged on the surface of a substrate of saidhead device with a specific pitch.
 5. A head device comprising: aplurality of head elements described in any one of claims 1 to 3 , whichare arranged on a substrate of said head device at a density of 25heads/cm² or more.
 6. A head element or a head device according to anyone of claims 2 to 5 , wherein said substrate is a conductive substrate.7. A head device comprising: a plurality of head elements havingmechanisms, said mechanisms allowing each of said head elements to beindependently moved a specific distance in the direction perpendicularto a substrate of said head device.
 8. A head element and a head deviceaccording to any one of claims 1 to 7 , wherein each of said headelements is driven a micro-distance in either or both of the X-axisdirection and the Y-axis direction by an electrostatic force orpiezoelectric effect.
 9. A memory medium in which an address signal isformed of an information signal to be two-dimensionally recorded on arecording plane.
 10. A memory medium in which an address signal isformed on a recording surface in the form of two-dimensionalirregularities.
 11. A memory medium in which a recording surface onwhich an information signal is to be recorded is divided intoinformation signal recording sectors of the same number as that of headelements of a head device for recording or reproducing an informationsignal described in any one of claims 4 to 8 .
 12. A memory medium inwhich irregularities corresponding to an information signal are formedon the surface by injection-molding or extrusion-molding.
 13. A memorymedium according to any one of claims 9 to 12 , wherein the substrate ismade from a conductive material or is a non-conductive substrate on thesurface of which a conductive thin layer is formed.
 14. A memory mediumaccording to claim 13 , wherein the recording surface is covered with aconductive carbon thin film or a diamond-like carbon thin film.
 15. Amemory medium comprising: a substrate flattened without irregularitiesor partially formed with irregularities, the surface of which is coveredwith a thin film, wherein said thin film is made from a materialallowing an impedance between said memory medium and a head element tobe locally changed before and after local application of an electricfield, current, heat or pressure by said head element.
 16. A method ofrecording/reproducing an information signal, comprising the steps of:making a head device face to a memory medium having a flat recordingsurface, said head device including a plurality of head elementstwo-dimensionally arranged each of which has at its leading end a flatportion having an area of 0.1 μm² or less; moving said head devicerelative to said memory medium a distance more than a gap between twoadjacent ones of said head elements; and recording an information signalat a specific position of said recording surface at a recording densityof 1 Gbit/cm² or more, or reproducing an information signal previouslyrecorded on said recording surface at a specific position by said headdevice.
 17. A method of recording/reproducing an information signal,comprising the steps of: sectioning a recording surface of said memorymedium described in claim 11 into sectors; and dividing an informationsignal of the same information signal series into parts and dispersedlyrecording the parts of the information signal on different ones of saidsectors in the form of irregularities at a recording density of 1Gbit/cm² or more.
 18. A method of recording/reproducing an informationsignal according to claim 16 or 17 , wherein an information signal isrecorded on the recording surface of said memory medium in the form ofirregularities or an information signal recorded on the recordingsurface of said memory medium in the form of irregularities isreproduced.
 19. A method of recording/reproducing an information signalaccording to claim 18 , wherein the information signal recorded on therecording surface in the form of irregularities is reproduced byapplying an electric field between said head element and the recordingsurface of said memory medium, and detecting an impedance between saidhead element and said memory medium, said impedance being changed due tosaid irregularities corresponding to the information signal,
 20. Amethod of recording/reproducing an information signal according to claim19 , wherein the information signal recorded on the recording surface inthe form of irregularities is reproduced by detecting an impedancebetween said head element and the recording surface of said memorymedium while bringing the leading end of said head element not incontact with the recesses of the irregularities but in contact with theprojections of the irregularities.
 21. A method of recording/reproducingan information signal according to any one of claims 16 to 18 , whereinthe information signal is reproduced by applying an electric fieldhaving a frequency higher than a mechanical primary resonance frequencyof a driving portion of said head device between the leading end of saidhead element and the recording surface of said memory medium, anddetecting a current modulated depending on the information signal.
 22. Amethod of recording/reproducing an information signal according to anyone of claims 16 to 21 , wherein an information signal is recorded orreproduced by moving said head device relative to said memory medium ina state in which part of said head elements of said head device face tothe recording surface of said memory medium.
 23. An apparatus forrecording/reproducing an information signal, comprising: a memory mediumincluding a flat information signal recording surface; a head devicearranged to be opposed to and in parallel to said information signalrecording surface of said memory medium, said head device including aplurality of head elements described in claim 1 two-dimensionallyarranged with a specific pitch; an applying device for applying anelectric field, current, heat or pressure between said informationsignal recording surface of said memory medium and each of said headelements; a driving device for moving said head device relative to saidmemory medium an extreme micro-distance; and a recording/reproducingcircuit for supplying an information signal to be recorded to each ofsaid head elements of said head device or taking out the informationsignal recorded on said memory medium by each of said head elements. 24.An apparatus for recording/reproducing an information signal accordingto claim 23 , wherein said applying device applies an electric fieldhaving a frequency higher than a mechanical primary resonance frequencyof said driving portion between each of said head elements of said headdevice and the recording surface of said memory medium.
 25. A method ofmanufacturing a micro-head element, comprising: a first step of forminga sacrifice layer made from a resist on a flat surface of a conductivesubstrate in such a manner that a central portion of said sacrificelayer is formed into a trapezoidal shape in cross section; a second stepof forming a metal film having a thickness of one to several μm over theentire surface of said conductive substrate in such a manner as to coverthe surface of said sacrifice layer; a third step of forming asymmetrical spring pattern on a central portion of said trapezoidalmetal film; a fourth step of forming a micro-resist film having one sideless than the thickness of said metal film on a central portion of saidspring pattern; a fifth step of etching said metal film directly beforesaid micro-resist film is perfectly separated from said metal film, toform a micro-head element including a leading end having a flat portion;and a sixth step of removing said sacrifice layer and said micro-resistfilm, to form a spring, made from said metal film, for elasticallysupporting said micro-head element.
 26. A method of manufacturing amicro-head element, comprising: a first step of forming an insulatingfilm having a specific thickness on the surface of a substrate which hasa conductivity at least on the surface; a second step of forming a maskon said insulating film at a specific position except for a portionequivalent to the cross-section of a columnar head element to bemanufactured; a third step of removing a portion of said insulating filmequivalent to the cross section of said columnar head element; and afourth step of forming a metal layer in the portion from which saidinsulating film is removed by plating, to form said columnar headelement.
 27. A method of recording/reproducing an information signal, inwhich an information signal is reproduced from a flat memory medium onwhich information is two-dimensionally arranged and recorded, by movinga head element, arranged to be opposed to and in parallel to theinformation signal recording surface of said memory medium, relative tosaid memory medium, comprising the steps of: two-dimensionally samplinga reproducing signal at an interval being a half or less of a pitchbetween signals two-dimensionally arranged on said memory medium;temporarily storing a specific amount of the data in a buffer memory;identifying, from said reproducing signal, an address signal astwo-dimensional positional information stored on said memory medium; andsignal-processing a time at which the address signal is reproduced, arelative speed between said head element and said memory medium, arelative moving direction between said head element and said memorymedium, and a data row stored in said buffer memory, thereby decodingthe content of the data at a specific position on said memory medium.